Instrument and method for recording mechanical transients

ABSTRACT

A recording accelerometer of the general type known as a read gage and a related method, such as is used in recording mechanical shocks in ballistics and impact studies. Inertial cantilever reeds of known natural frequency each carry roundpointed styli, arranged to scratch or emboss line-like marks on smooth, relatively soft recording surfaces. The shape and pressure of the stylus tips is such that they produce some plastic displacement of the recording surfaces. It is found that the limits of the vibratory excursions of each reed are visible under a microscope, marking the peaks of each half-cycle of such oscillations. Since these peaks occur at known time intervals (the natural frequencies being known), time information is obtained from the record in addition to the usual displacement information. Thus the entire decaying oscillatory waveform executed by each reed can be recovered from the &#39;&#39;&#39;&#39;scratch&#39;&#39;&#39;&#39; records, even though the recording medium does not move to provide a time base in the usual sense. From this waveform, a curve of force or acceleration vs. time, or shock signature, can be plotted.

SR XR 396839397 uulwtl D1 Engdahl [54] INSTRUMENT AND METHOD FORRECORDING MECHANICAL TRANSIENTS [72] Inventor: Paul D. Engdahl, 2850Monterey Ave., Costa Mesa, Calif. 92626 [22] Filed: Aug. 14, 1970 [21]Appl. No.: 63,752

[52] US. Cl. ..346/l, 73/70.2, 346/7, 346/77, 346/135 [51] Int. Cl...G0ld 9/02 [58] Field of Search ..346/7, 77,111,134,135,1;

[56] References Cited UNlTED STATES PATENTS 2,906,117 9/1959 Kennard..73/70.2

3,092,434 6/1963 Uinet ..346/107 3,445,857 5/1969 Pope 346/7 3,479,03611/1969 Carroll et al ..274/4 Aug. 8, 1972 Primary Examiner-.loseph W.l-lartary Att0rneyLawrence Fleming [57] ABSTRACT A recordingaccelerometer of the general type known as a read gage and a relatedmethod, such as is used in recording mechanical shocks in ballistics andimpact studies. lnertial cantilever reeds of known natural frequencyeach carry round-pointed styli, arranged to scratch or emboss line-likemarks on smooth, relatively soft recording surfaces. The shape andpressure of the stylus tips is such that they produce some plasticdisplacement of the recording surfaces. lt is found that the limits ofthe vibratory excursions of each reed are visible under a microscope,marking the peaks of each half-cycle of such oscillations. Since thesepeaks occur at known time intervals (the natural frequencies beingknown), time information is obtained from the record in addition to theusual displacement information. Thus the entire decaying oscillatorywaveform executed by each reed can be recovered from the scratchrecords, even though the recording medium does not move to provide atime base in the usual sense. From this waveform, a curve of force oracceleration vs. time, or shock signature, can be plotted.

4 Claims, 10 Drawing Figures PATENTED 9 I972 3. 683,397

sum 1 or 2 fwsmme MWM BACKGROUND OF THE INVENTION In engineering studiesof transient mechanical phenomena such as impacts and explosions, it iscommon practice to record the acceleration experienced at some locationon a moving object or on a target. In research work, recordings are madeof other transient mechanical forces, such as may be derived from theair blast of an explosion or from the magnetic field produced by a pulseof electric current, e.g., a lighting stroke.

Such data are often recorded by means of mechanoelectric transducerssuch as pressure pickups or accelerometers, plus wire or radio telemetryand tape recorders or oscillographs. On certain occasions,

however, such instrumentation is not suitable, as where adequate spaceand electric power are not available, or where extreme ruggedness orreliability are required, or the time of occurrence of the phenomenon isnot predictable and may be some weeks or months after theinstrumentation is installed. For such occasions, simpler,self-contained instruments are used, of types which require no electricpower. They are also used as backup instruments in, for example, missiletesting, in case of failure of the more elaborate instrumentation.

The present invention relates to simple, self-contained instruments ofthis class. One such instrument is generally known as an impactaccelerometer or reed gage. Its main components are a mechanical mass-'spring system having substantially a single degree of freedom, orresonant mass-spring system, together with a stylus attached thereto,and a recording surface. This surface may be a thin plating of gold on asteel backing. When the instrument as a whole is accelerated by anexternal mechanical shock, the recording surface moves with respect tothe stylus, and the peak acceleration is recorded as a scratch on therecording surface. The displacements are usually small in practice, andthe scratch amplitude is measured under a microscope. The natural orresonant frequency of the resonant mass-spring system being known, thedisplacement per unit acceleration is also known. Several such resonantsystems, usually in the form of reeds, are often used in one instrument.

Prior instruments of this general class are of two types. The first hasa stationary recording surface, so that when the stylus moves back andforth, it retraces substantially the same path. In such prior artinstruments, only the maximum excursions of the stylus can be read fromthe record. The second prior type uses a recording surface in the formof a disc, drum, or tape, which is moved by motor means to provide atime base. Here, the whole waveform executed by the stylus is recorded,usually in the form of a decaying oscillation; and from it the timehistory of the acceleration force (or other mechanical force beingrecorded) can be recovered. Such a time history of acceleration issometimes called a shock signature. It is much more valuable than a mererecord of the peak acceleration value. For example, a body acceleratedat a rate of 1,000 g for l millisecond will travel about 0.2 inches, andsuch a deflection might be absorbed elastically by a structure; but ifthis acceleration lasts for milliseconds, the distance is about inches.

The instrument of this invention uses no motor means to provide a timebase, but does provide a novel way for the recovery of the accelerationvs. time curve or shock signature.

BRIEF SUMMARY OF THE INVENTION A preferred form of the present inventioncomprises a number of resonant mass-spring system or passive mechanicaloscillators in the form of resonant reeds of different naturalfrequencies, each tipped with a stylus and disposed so that the stylusis pressed against a soft smooth metal surface, and will scratch oremboss the surface in proportion to the deflection of its reed. Thestyli have hard, smooth, rounded tips and are preferably of diamond. Thetip radius is preferably somewhat less than that of a phonograph pickupstylus. The recording surface may be a bright gold flash of the order of30 microinches thick plated over a polished substrate of stainless steelor the like, followed by a flash of gold'plated at a faster than normalrate to provide a relatively coarse brownish deposit. The latter type offlash or plating is known as smut gold.

With such a structure, the stylus produces an appreciable plasticdefonnation of the gold surface in its path, throwingup small ridges oneither side, and pushing away and piling up material at the ends of eachof its strokes. The action is somewhat like that found in the dry pointprocess in fine arts. There appears to be no established term thatdefines the action exactly. Hence the term embossing is used here inreference to it. The word is used in approximately the sense in which itwas formerly applied in sound recording. An illustration of such usageis found in the book The Recording and Reproduction of Sound" by OliverRead, published by Howard W. Sams & Company, 1952 (page 21).

The piling up of gold at the ends of the vibratory excursions of thestylus, during this embossing process, provides peak-amplitude marks foreach half-cycle which are visible under a microscope. By measuring andplotting the locations of each such mark, the peaks of each of theseveral or many excursions of the stylus may be connected on paper, togive the envelope of the decaying mechanical oscillations. The mediansof these excursions may then be plotted and then connected by a drawnline. These median points are points defining the acceleration vs. timehistory of the stylus, i.e., the shock signature. The time scale isknown because the time between successive peaks is known: half thenatural period of the passive mechanical oscillator or reed. Thus thedesired time-base data is recovered from a simple instrument having astationary recording medium.

The same principle may be applied to any means of recording wherein theends of the successive vibratory excursions of the stylus, or othermarking device, can be detected. Embodiments of the invention are shownherein which use magnetic recording and photographic recording.

SHORT DESCRIPTION OF DRAWING FIG. 1 is a simplified top view, with thecover removed, of an eight-reed recording accelerometer or reed gageaccording to the invention;

and recording means of a modification of the invention,

using magnetic recording;

FIG. 7 is a diagram of an alternative type of magnetic head;

FIG. 8 is a diagram of a magnetic playback apparatus;

FIG. 9 is a graph showing two types of display record from the playbackapparatus of FIG. 8;

FIG. 10 is a partial diagrammatic perspective view of a furthermodification of the invention, using photq graphic recording.

DETAILED DESCRIPTION In the simplified top view of a recordingaccelerometer, FIG. 1, a series of eight reeds is shown, 10 10g. At thetip of each reed is a stylus carrier 12, to the end portion of which isattached a stylus 14. The tip of each stylus bears upon one of therecording surfaces 15 of a hub 16. Eight surfaces such as 15 are shownin FIG. 1, each preferably with a soft coating such as a thin plating orflash of smut gold (described above) over a flash of bright soft gold.

In FIG. 1, the reeds l0 10g may each have a different natural frequency,and hence a different deflection sensitivity, so that usable records canbe obtained over a wide range of accelerations, and data obtained for ashock spectrum (defined in the literature). Each reed may be a portionof a single piece of sheet spring material 17, which may be machined,stamped, or etched to the required shape. The center hub 16, whichcarries the records, is removable.

When the hub 16, FIG. 1, is installed in the instrument case, it islowered into place with the recording surfaces such as 15 opposite thestylus carriers 12; then it is rotated through a small angle (indicatedby arrow 40) so that the styli are approximately centered on therecording surfaces 15. During this rotation, each stylus makes ahorizontal scratch, which provides a zero reference line for thesubsequent record. Such a scratch is shown at 43 in FIGS. 2 and 4. Whenthe hub is removed from the instrument, it is again rotated in the samesense until the styli 14 are opposite the depressed areas such as 27,and then lifted out of the instrument. During these rotations, the stylileave scratches such as indicated at 41, FIGS. 2 and 4. Both thesescratches normally line up. If they do not, it is an indication that thezero position of the reed has shifted during use, as by overstressing.

FIG. 2 shows the structure associated with one of the reeds 10 to alarger scale and in more detail. When the whole instrument is subjectedto an acceleration having a component in the direction of the arrow 22,an inertial force, indicated by arrow 23, will act to bend the reed 10as indicated by arrow 24. Stylus carrier 12, at-

records for microphotography is difficult.

tached to the tip portion of reed 10, acts as a spring to press thestylus 14 against the recording surface 15. It is made much stifi'er inthe direction of arrow 24 than in the direction of the axis of thestylus 14. When the stylus carrier moves with respect to the recordingsurface 15, the stylus 14 scratches or embosses a record mark 19thereon.

The base portion of reed 10, which is part of the spring sheet 17, maybe clamped between the adjacent portion of the frame 25 and a clampingmember 26, as shown in FIG. 2. Centerpiece or hub 16, a portion of whichis shown in FIG. 2, is removably attached to frame 25.

Proceeding now to FIG. 3, a greatly enlarged sectional view illustratesthe general nature of the marking process. The stylus 14 may be ofdiamond, with a spherical tip 28 of the order of 0.0008 inch radius. Therecording surface 18 may comprise a thin layer of smut gold 20 over agold flash 90. Underlying this may be a layer of plated nickel 29 on thesubstrate 16, which may be of stainless steel or other suitablestructural material. When the stylus 14 moves, its tip portion 28 digsor embosses a groove 19 in the recording surface. At the end of astroke, the gold 90, 20 is pushed or piled up into a ridge 1. Such aridge is visible under a microscope, and marks the end or peak of anoscillation or stroke.

It will be understood that in practical instruments of this nature, thedimensions of the record are small. The mark such as 19 may be 0.1 inchlong and about 0.001 inch wide. It is usually read with a microscope of-400 power with a micrometer stage. The ridges or end marks such as 1appear as dark cross-bar s at a magnification of 100X. At 400X they looklike lumpy piles of material resembling slightly a heap of soil leftbehind by a retreating bulldozer blade. Sectioning of such Themicrophotographs from which the above description was made were taken asvertical views of the record.

While I have indicated stylus and recording surface dimensions andmaterials specifically, it is understood that any kind of stylus ormarking element, and any record surface, which will render visible ordetectable the peaks of the cyclic linear excursions of the stylus fallwithin the purview of the invention. For example, sapphire or steelstyli with recording surfaces of suitable plastics may be used. Magneticand photographic recording means are described later.

FIG. 4 shows diagrammatically the nature of a scratc or embossed recordaccording to the invention. The broadly curved double line 19 indicatesthe boundaries of the record mark, and corresponds to a diagrarmnaticenlarged view of the mark 19 in FIG. 2. As the reed 10, FIG. 2, isexcited into oscillation at its natural frequency by a suddenacceleration or deceleration, its oscillations decay, damped by thecoulomb friction presented to the stylus. An initial excursion of thestylus ends or peaks, in FIG. 4, at 1. The next swing ends at 2, and thenext half-cycle of oscillation at 3, the following half-cycle peak at 4,and so on until the amplitude becomes relatively small at 9. Theasymmetrical distribution of these peaks about the zero axis indicatesthat a driving force (in this case, an acceleration) was present, andchanging relatively slowly with respect to the natural period of thereed 10. The

time interval At between peak marks 1, 2, 3, 4, etc. is obviously equalto half the natural period of oscillation of the reed 10.

FIG. 5 shows a graphic plot of the amplitude and time informationobtained from FIG. 4 and from knowing the natural frequency of the reed10. Point 1 is plotted at a positive X-displacement analogous to theamplitude of point 1 in FIG. 4, point 2 analogous to 2, and so on, attime intervals At equal to half the natural period of oscillation of thereed.

These points 1, 2, 3 9 may be connected by a line 54 in the shape of adecaying sinusoid, to depict the actual waveform of the motion of thestylus, as shown in FIG. 5. This, however, is useful primarily forillustrative purposes. To reconstruct the acceleration vs. time curve orshock signature, from the plot of points 1-9, it is preferable to draw aline 50 through all the upper points 1, 3, 5, and a line 51 through allthe lower points 2, 4, 6, While lines 50, 51 are shown as substantiallystraight in FIG. 5, they may obviously take whatever shapes are dictatedby the points 1-9, such as exponential curves. These lines 50, 51 definethe excursion envelope of the record.

The shock signature may now be plotted from the excursion envelope, asits median. From each point l-9 a vertical line may be drawn, tointersect the opposite side of the excursion envelope, and the midpointof that line plotted as a point on the shock signature. For example,vertical 52 extends from point 2 across the excursion envelope, betweenlines 51 and 50. The midpoint of line 52 is plotted as point [2.Similarly, a vertical from point 3 to line 51 provides point c at itsmiddle, and so on, the shock signature being shown as the heavy line inFIG. 5 connecting points b i.

The portion of the shock signature between the starting point 0 (on baseline 43, 41) and point b, opposite the second excursion peak 2, is showndotted, because it is not always determined in as simple a manner as thesubsequent points. If the applied shock acceleration rises rapidly incomparison to the half-period of oscillation of the reed, At, theinitial portion of the record O-b may be of a more complex nature thanthe example shown in FIG. 5, and require more complex methods for itsreduction. Such methods do not form a part of the invention.

It will be seen that the excursion envelope has been determined by thepositions of the peak marks 1, 2, 3, in the record of FIG. 4, and fromknowing the natural period 2A! of the reed; and that the excursionenvelope 50, 51 determines the curve of acceleration vs. time, or shocksignature 53. The necessary time information has been obtained from thenatural period of the reed, without any need for physically moving therecord medium with respect to the stylus, as by means of a motor.

Referring back to FIGS. 2 4, it will also be apparent that such recordsmay be obtained by this invention from any driving force 23 (FIG. 2)from any source, such as a magnetic field or a blast of gas, providedthat the source of this force has a mechanical impedance such that itdoes not excessively load the resonant netic recording instead ofmechanical embossing or the like. FIG. 6 is a partial, enlarged,perspective view of a reed according to this form of the invention,showing means to record its motion magnetically. The reed is shown at10, and may be similar to the reed 10 in FIG. 2, and the reeds 10, 10a.10g in FIG. 1. It may carry a resilient carrier 12' at its end portion,which may be similar to the stylus carrier 12 in FIG. 2. In place of astylus at the end portion of carrier 12, however, there is provided amagnetic element 61. This may be in the form of a small permanent barmagnet with a pointed or chisel-shaped tip, as shown. Alternatively, itmay be of the kind shown at 62 in FIG. 7, a small magnetic head shapedfor conventional longitudinal magnetization of the medium, with apermanent magnet 69. Both the magnet 61, FIG. 6, and the head 62, FIG.7, are disposed adjacent or touching a magnetic record surface 60. 1

Magnetic record surface 60 may be of any suitable type, such as aconventional coating of particles of magnetic oxide in a binder, appliedto a non-magnetic substrate, not shown in FIG. 6. The substrate may be asurface 15 on hub 16, FIGS. 1 and 2. Alternatively, a piece of magneticrecording tape may be cemented to surfaces 15 as in FIG. 2.

A known method of producing a retrievable record on a magnetic medium isto pre-record an unmodulated carrier wave of relatively high frequencyon the medium, and then employ a movable permanent magnet markingelement or head to erase, or to partially erase, this carrier. Themechanical displacements of the permanent magnet device then leave aretrievable record on the medium in the form of absences or reductionsin the pre-recorded carrier. It is also known that such a movablepermanent magnet marking element may have its field strength chosen sothat it only partially erases the carrier on a single pass across themedium; subsequent passes then reduce the carrier wave magnetization tosuccessively greater degrees.

Accordingly, the invention may be embodied in a pre-recorded carrierwave system as shown in FIGS. 6-9. In FIG. 6, the carrier wave may bepre-recorded (by obvious means not shown) to provide a magnetic trackindicated at 74 on record medium 60. When the reed 10' executes adecaying oscillatory motion, the small permanent magnet 61 moves backand forth along the track 74, producing successively greater erasurewith each pass. The resulting record can be retrieved in various ways.Magnetic records can be made visible, as is known, by applying thereto asuspension of fine magnetic particles in a volatile liquid. The ends orpeaks of each oscillatory pass of the magnet 61 can then be made visible(under optical magnification), by particle patterns representingstep-like changes in the intensity of magnetization.

In the mechanical record of FIG. 4, the portion of the record betweenpoints 1 and 3, and between points 2 and 4, has been passed over twice;the portion between points 3-5 and 4-6 has been passed over four times,and so on. Thus the excursion peaks such as 1-9 in FIG. 4 can belocated. In a magnetic record, the same process applies, wherever theexcursion peaks of the marking device can be located. The shocksignature can then be derived and plotted, in the same way as describedin connection with FIG. 5.

Another method of data retrieval is indicated in FIGS. 8 and 9. Insteadof using magnetic particles (dusting), the record medium 60 maybe playedback I by moving it under a conventional magnetic playback head 63, FIG.8. The output of playback head 63 may be fed into an amplifier 64 and adetector 65, thence to a storage oscilloscope 66 or other suitabledisplay instrument. Referring now to FIG. 9, the intensities ofdemagnetization (or magnetization) along the record track 74 (FIG. 6)may be illustrated graphically at 70. On line 70, the first excursion ofthe magnet or head 61 or 62 was between points 1' and 2'. The originalmagnetic state of the track is indicated as level M, M. Between points1' and-3', and 2 and 4, the state is changed by a certain increment asat 72, 72'. Between points 3 and 5, and 4' and 6, it is changed by afurther increment since the magnet 61 or 62 has made two more passesover this portion of the track and so on. Thus the excursion peaks 16'can be displayed on the instrument 66. It is assumed that the output ofdetector 65 is filtered, so that the frequency of the pro-recorded waveitself does not appear in the display.

In certain systems it is advantageous to use a flat, i.e.,non-integrating, amplifier at 64, so that the signal fed to display 66is in the same differentiated form supplied by the playback head 63. Thedisplay then is in the form shown at 73 in FIG. 9. The excursion peaks16 now appear as their time derivatives, the pips l"6. When thereversal-of the reed displacement is in one sense, the pips are upward;in the opposite sense, downward. Thus the rest position of the magnetdevice 61 or 62, i.e., zero line 71 (FIG. 9), may be easily identified,being between the last upward pip, as 6", and the last downward pip, as5". Such pips provide clear indications of the locations of theexcursion peaks, and this information, together with the naturalfrequency of the reed, may be used to obtain the shock signature oracceleration vs. time curve, in the same manner as described inconnection with FIGS. 4 and 5.

FIG. 10 shows a further modification of the invention, usingphotographic recording. Reed 100, which may be similar to reeds 10 and10' of FIGS. 2 and 6, carries a source 85, from whose tip portion 86light or other radiation is emitted, in a fine beam of very smallcross-sectional area. The record medium 80, adjacent tip 86, is a pieceof photographic film or the like. Source 85 may emit any 'kind ofradiation that will suitably effect a photographic medium. Since theoptical density of exposed film is approximately proportional to thelogarithm of the exposure, and the exposure is proportional to thevelocity with which the source 85 is moving with respect to the film 80,it is evident that an exposed track 84 will be produced having densitygraduations which show the excursion peaks of the decaying oscillatorymotion of the reed 100.

In this specification, the term, resonant mass-spring system" means anymechanically resonant element or structure having mass and elasticitywhich can be used to provide a substantially known deflection inresponse to an applied force. The term marking element is used to meanany element whose presence leaves a detectable mark or record on therecording surface, e.g., %il8li a8iil%ll lli% 3123.333 f r rl gfiitihead on a surface of magnetic recording material.

The invention is not limited to the specific modes of recording heredescribed, but may include any method of recording within the scope ofthe claims.

lclaim:

1. A method for determining the waveform and magnitude of a transientmechanical force, comprising the following steps:

a. applying said force to a resonant mass-spring system of known naturalperiod to produce a displacement of a point on said system having atransient component and a decaying oscillatory component,

b. recording said displacement along substantially a single line,

c. detecting the peak excursions of said oscillatory component at pointsalong said line,

d. plotting the displacements of said peaks at intervals along a timeaxis determined by said natural period, and

e. plotting the mean displacements between said peaks to provide pointsof transient displacement along said time axis to recover said waveform.

2. A method as in claim 1,

said force being the inertial reaction of the mass of said system to anapplied acceleration.

3. .The method of claim 1, wherein,

said recording step includes embossing with a rounded stylus on arelatively soft recording surface.

4. A recorder for a transient mechanical force, comprising:

a plurality of resonant mass-spring systems responsive to said force andeach having a known natural frequency,

a portion of each said system being adapted to execute a displacementhaving a transient component substantially proportional to said forceand a decaying oscillatory component at its natural frequency,

said displacement being along a known line;

a marking stylus having a hard rounded tip connected to each saidportion; and

a softer recording surface in marking relation with each said stylus toreceive an embossed mark along said line with substantial plasticdeformation and flow of said surface, to make detectable the individualpeak excursions in each half-cycle of said oscillatory component,

said mass-spring systems being disposed generally tangentially around acentral hub and said recording surfaces being on said hub and separatedby relieved portions; and

means for rotating said hub to bring one of said relieved portionsopposite each said stylus to permit installation and removal of saidhub,

said transient component with its time-scale being recoverable fromknowledge of the locations of said peak excursions and of said naturalfrequencies.

1. A method for determining the waveform and magnitude of a transientmechanical force, comprising the following steps: a. applying said forceto a resonant mass-spring system of known natural period to produce adisplacement of a point on said system having a transient component anda decaying oscillatory component, b. recording said displacement alongsubstantially a single line, c. detecting the peak excursions of saidoscillatory component at points along said line, d. plotting thedisplacements of said peaks at intervals along a time axis determined bysaid natural period, and e. plotting the mean displacements between saidpeaks to provide points of transient displacement along said time axisto recover said waveform.
 2. A method as in claim 1, said force beingthe inertial reaction of the mass of said system to an appliedacceleration.
 3. The method of claim 1, wherein, said recording stepincludes embossing with a rounded stylus on a relatively soft recordingsurface.
 4. A recorder for a transient mechanical force, comprising: aplurality of resonant mass-spring systems responsive to said force andeach having a known natural frequency, a portion of each said systembeing adapted to execute a displacement having a transient componentsubstantially proportional to said force and a decaying oscillatorycomponent at its natural frequency, said displacement being along aknown line; a marking stylus having a hard rounded tip connected to eachsaid portion; and a softer recording surface in marking relation witheach said stylus to receive an embossed mark along said line withsubstantial plastic deformation and flow of said surface, to makedetectable the individual peak excursions in each half-cycle of saidoscillatory component, said mass-spring systems being disposed generallytangentially around a central hub and said recording surfaces being onsaid hub and separated by relieved portions; and means for rotating saidhub to bring one of said relieved portions opposite each said stylus topermit installation and removal of said hub, said transient componentwith its time-scale being recoverable from knowledge of the locations ofsaid peak excursions and of said natural frequencies.